Fan Stall Inhibitor

ABSTRACT

An impeller blade hub profile which inhibits stall is described. In an embodiment, the hub profile includes a frontal rising contour followed by a rear constant contour. In another embodiment, the hub profile includes a frontal rapidly rising contour followed by a substantially rear constant contour. It was discovered that this combination is much more effective than conventionally known profiles. Also, the present invention provides a unique way of turning the flow within the passages of the rotating impeller blades. In an embodiment, the blade does not turn much over either the front portion of the hub or the rear portion of the hub, but instead turns near the boundary between the front portion and the rear portion.

BACKGROUND OF THE INVENTION

Embodiments of the present invention relate to fans, and in particular to a hub design configured to retard or otherwise inhibit fan stall.

FIG. 8 shows a conventional fan 800. A conventional fan typically includes a housing 802, a rotating impeller 804 comprising a plurality of blades 806 disposed on a hub 808. Typically, a cavity inside the impeller hub 808 houses the motor, motor stator, drive, and bearings (not shown). FIG. 9 shows plan view and a top view of a typical fan impeller 900 with blades 902 attached to the hub 904. The exterior side-surface of the hub 904 to which the blades are attached can be referred to as the “impeller hub contour”; it is the contour of the outer surface of the hub. The perimeter defined by rotation of the blades 902 can be referred to as the “impeller tip perimeter.”

The function of the fan is to capture, pressurize (by action of the rotating blades), and deliver air. This task is accomplished via the impeller blades rotating about an axis. Rotation may be provided by a motor, for example. To pressurize air, two elements must be present: the first is blades (whose shape/geometry is design-dependent), and the second is frequency (or rotational speed, radian/sec; externally induced).

While flowing across the blades (from inlet to outlet), pressure increases due to two basic mechanisms: first, the flow is forced to continuously turn along the curved surfaces of the blades; and second, the flow streamlines tend to naturally migrate into higher radii (and thus higher impeller speeds). But sometimes these two flow processes, (1) turning due to blade camber and (2) centrifuging effect, do not always produce coherent flow over all blade surfaces.

While the impeller is rotating, the flow may, under certain operating conditions, separate off the suction surface of some of the blades. Referring to FIG. 9, the “suction” surface of a blade 902 is the upper surface of the blade viewed from the direction from the inlet side; the bottom surface of the blade is referred to as the “pressure” surface. The separation of flow off the suction surface of the blade leads to a reduction in pressure; this condition is called stall. Under normal operating conditions, the flow remains attached to all the blades all the time, and the blades are doing work efficiently. However, stall will occur when at least some of the flow, somewhere between the impeller hub contour and the impeller tip perimeter, is no longer able to remain attached to all the blades; in this case, the stalled portions of the blade surfaces become ineffective.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the invention provide an impeller comprising a hub having a unique hub contour. In an embodiment, the hub has a hub contour (hub profile) that rises very quickly along the length of the frontal portion of the hub and does not substantially change over length of the rear portion of the hub. In another embodiment, the hub contour results in two cross-sectional-area zones along the entire flow path: a frontal rapidly shrinking zone; and a rear constant area zone. In an embodiment, an impeller includes a hub comprising a first part that has a varying hub contour and a second part that has a substantially non-varying hub contour. One or more fan blades of the impeller are attached to the hub in a manner that each such fan blade attaches to the first part of the hub and to the second part of the hub. In an embodiment, the length of the first part of the hub constitutes at least a quarter of the total length of the hub. These and other embodiments are described in further detail below with reference to the figures which will now be briefly described.

BRIEF DESCRIPTION OF THE DRAWINGS

The figures are merely diagrammatic, illustrative representations of embodiments of the present invention, and as such the illustrated structures are not necessarily to scale. The figures should not be construed as design specifications for the construction of embodiments of the present invention.

FIG. 1 illustrates an embodiment of a fan assembly in accordance with present invention.

FIG. 2 illustrate a cross-sectional view of an embodiment of a fan assembly in accordance with the present invention.

FIG. 3 shows a top view of an embodiment of an impeller according to the present invention.

FIGS. 3A and 3B are schematic profiles of an embodiment of a hub according to the present invention.

FIG. 4 shows a schematic cross-sectional view of a hub according to the present invention.

FIG. 5A is a schematic illustration of an impeller according to the present invention.

FIG. 5B is a top view of the rendering of FIG. 5A.

FIG. 6 is a cutaway view of a portion of the rendering of FIG. 5A.

FIG. 7 is a schematic representation of an alternate profile of a hub in accordance with the present invention.

FIG. 8 illustrates a conventional fan assembly.

FIG. 9 illustrate two views of a conventional impeller.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 depicts a fan 100 embodied in accordance with the present invention. The figure is merely a diagrammatic, illustrative representation of an embodiment of the present invention, and as such the illustrated structures are not necessarily to scale. The fan 100 may include a fan housing 102. The fan housing 102 can be provided with mounting holes 103 for mounting the fan 100 to a device to be cooled by the fan. An impeller 104 according to the present invention is disposed within the fan housing 102. The impeller 104 comprises a hub 106 embodied in accordance with the present invention, and fan blades 108 disposed on the hub in accordance with the present invention. The figure shows arrows indicating the direction of airflow, entering the fan 100 at an air inlet side and exiting the fan at an air outlet side.

FIG. 2 shows a cross-sectional view of the fan 100 depicted in FIG. 1. The figure is merely a diagrammatic, illustrative representation of an embodiment of the present invention, and as such the illustrated structures are not necessarily to scale. The cross-sectional view shows that the hub 106 can incorporate a fan motor 202. The motor 202 may include a canister-like containment referred to as the yoke 204. The yoke 204 is connected to the interior volume of the hub 106. A shaft 212 is attached to the yoke 204 allowing the impeller 104 to rotate about an axis of rotation established by the shaft. A magnetic rotor element 206 may be connected to the inside wall of the yoke 204. A magnetic stator element 208 is fixedly mounted within the volume of space of hub 106 along the axis of rotation. In an embodiment of the present invention, the motor 202 may be a DC brushless motor.

FIGS. 3, 3A, and 3B illustrate an embodiment of an impeller 302 according to the present invention. The figures are merely diagrammatic, illustrative representations of an embodiment of the present invention, and as such the illustrated structures are not necessarily to scale. The impeller 302 is viewed from the front (inlet side), looking toward the outlet side. In an embodiment, the hub 304 provides two successive flow zones: First, is a frontal zone 304 a, where the flow area decreases with distance from the inlet side of the hub toward the outlet side of the hub up to a distance D. Second, is a rear zone 304 b where the cross-sectional flow area is substantially constant from distance D on the hub toward the outlet side for the remaining length of the hub. Reference letter P represents the perimeter of the area swept by rotation of the fan blades 306. The flow area is the area between a surface of the hub 304 and the perimeter P.

FIG. 3B illustrates the decreasing flow area characteristic of the frontal zone portion of the hub 304. The hub 304 is displayed in four panels, each shown in cross-section with its profile exaggerated to facilitate illustration of the decreasing flow area of the frontal zone 304 a. The airflow direction is indicated and the inlet side and the outlet side of the hub 304 are shown. The first panel (1) shows a cross-section of flow area A₁ at the front end of the hub 304, designated as location L₁ on the hub. The flow area “seen” head-on by the airstream is also shown. The perimeter P, as explained in FIG. 3B, represents the area swept by rotation of the blades. The portion of the hub 304 “seen” by the air stream is illustrated by the hatched circle. The area A₁ is greatest at the front end of the hub 304. The second panel (2) shows a cross-section of flow area A₂ along the axial location of the hub 304, designated as L₂, that is toward the outlet side of the hub. The flow area A₂ is less than the flow area A₁, due to the increase in the area of the hub “seen” by the airflow. The third (3) panel shows a cross-section of flow area A₃ along the axial location of the hub 304, designated L₃, that is further still toward the outlet side of the hub. The flow area A₃ is less than the flow area A₂. This progressing decrease in flow area continues until the airflow reaches axial location D on the hub 304. Thus, in panel (4), at axial location D, the flow area A₄ remains substantially constant along the remaining portion of the hub 304 beginning at location D in the direction toward the outlet side of the hub.

Due to the continuously rising hub, the frontal area A decreases along the entire axial-length of the frontal zone 304 a of the hub 304. This causes pressure to be produced via the centrifugal effect (streamlines migrate into higher radii). Over the axial-length of the rear zone of the hub 304, pressure is produced via flow turning due to blade camber (i.e. curvature). The total pressure-rise is the sum due to the two distinct mechanisms (centrifugal effect plus flow turning).

Referring to FIG. 4, a cross-sectional view of a hub 404 in accordance with the present invention is shown. The figure is a diagrammatic, illustrative representation of an embodiment of the present invention, and as such the illustrated structures are not necessarily to scale. The cross-sectional view can be referred to as the “hub profile” or the “hub contour” (outer surface of the hub to which the fan blades are attached). Physical features of the hub profile illustrated in the figure are exaggerated to facilitate illustrating aspects of the present invention. In an embodiment of the hub 404, the front of the hub can extend further than is illustrated in the figure; this is indicated by the dashed outline 404 a.

FIG. 4 shows an axis of rotation; a counterclockwise rotation is shown as an example. In later discussions, the axis of rotation lies on the Z-axis of a cylindrical coordinate system. The direction of airflow is indicated in the figure, where a flow of air enters at the inlet side and exit from the outlet side. The inlet (upstream) side of the hub 404 can also be referred to as the hub leading edge (hub LE). The outlet (downstream) side of the hub 404 can be referred to as the hub trailing edge (hub TE).

In an embodiment, the hub 404 comprises a first portion 406 a (corresponds to frontal zone 304 a) and a second portion 406 b (corresponds to rear zone 304 b). The first portion 406 a can be characterized as having a rising hub contour (RHC) in that the radius, r, of the hub 404 varies along the axial length of the first portion. The radius is the distance measured from the axis of rotation to the outer surface (hub contour) of the hub 404. In FIG. 4, radii r₁-r₅ are examples of radius measurements of the hub contour along the length of the axis of rotation, measured from the axis of rotation to the outer surface of the hub 404. In an embodiment, the radius of the first portion 406 a of the hub 404 increases in the axial direction from the hub leading edge toward the hub trailing edge. FIG. 4 shows two examples of radii r₁ and r₂ of the first portion 406 a taken along the axis, where r₂>r₁. The second portion 406 b of the hub 404 can be characterized as having a constant hub contour (CHC) in that the radius of the hub does not substantially vary along the axial length of the second portion. FIG. 4 shows three examples of radii r₃, r₄ and r₅ of the second portion 406 b taken along the axis, where r₃ is substantially equal to r₄ which is substantially equal to r₅.

In an embodiment, the hub 404 can be further characterized by a total axial length, L. The axial length of the first portion 406 a can be represented by L₁ and the axial length of the second portion 406 b can be represented by L₂, where L=L₁+L₂. The figure also shows a leading edge portion 416 a of the hub 404, a trailing edge 416 b of the hub, and a middle portion 416 c of the hub. The leading edge portion 416 a is a “front part” of the first portion 406 a of the hub 404. The trailing edge portion 416 b is a “rearward part” of the second portion 406 b of the hub 404. These portions of the hub are discussed further below.

FIG. 4 shows the RHC-CHC boundary disposed between the hub leading edge end of the hub and the hub trailing edge end of the hub. The RHC-CHC boundary need not be sharp, angled, transition as shown in the figure. In embodiments of the hub, the transition at the RHC-CHC boundary can be a curved, smooth, or continuous transition.

A more formalistic description of embodiments of the present invention will now be discussed with reference to FIGS. 5A-6. FIGS. 5A-6 are merely diagrammatic, illustrative representations of an embodiment of the present invention, and as such the illustrated structures are not necessarily to scale. FIG. 5A is a schematic representation showing in profile an impeller 502 embodied in accordance with the present invention.

In FIG. 5A, the direction of the airflow is shown, where the flow enters on the inlet side then exits at the outlet side of the impeller 502. The axis of rotation is shown and a direction of rotation of the impeller 502 is indicated. The camber line is the mean line of the blade profile. The camber line extends from the leading edge to the trailing edge, halfway between the pressure side and the suction side. The impeller 502 comprise the hub (impeller hub) 504 and fan blades 506. One of the fan blades is shown in cross-section.

The hub 504 comprises a first portion 504 a that is characterized with a rising hub contour and a second portion 504 b that is characterized by a constant hub contour, as defined and explained above. One of ordinary skill will appreciate that the hub 504 can be manufactured as single piece, for example, by an injection mold process where the first and second portions 504 a, 504 b are formed in the same step. The hub 504 can be manufactured as two pieces in separate manufacturing steps and then connected together. For example, the first piece can be the first portion 504 a and the second piece can be the second portion 504 b, which can then be connected together to form the hub 504. These and other manufacturing steps can be used.

Referring for a moment to FIG. 5B, a top view of the impeller 502 looking in the downstream direction is shown. Each fan blade (e.g., 506 a) has a portion thereof (referred to herein as the leading edge portion) that is connected to, attached, or otherwise formed onto at least part of the first (RHC) portion 504 a of the hub 504. The figure represents this connection as 516. In addition, fan blade 506 a has another portion (referred to herein as the trailing edge portion) thereof that is connected to, attached to, or otherwise formed onto at least part of the second (CHC) portion 504 b of the hub 504. The figure represents this connection as 518.

Referring now to FIGS. 4, 5A, and 5B, a discussion about “blade turning” in accordance with the present invention will now be presented. For each blade 506, the leading edge portion of the blade has a blade angle of β₁ (also referred to as the blade inlet angle), which is generally illustrated in FIGS. 5A, 5B. Likewise, the trailing edge portion of the blade 506 has a blade angle of β₂ (also referred to as the blade exit angle), which also generally illustrated in FIGS. 5A, 5B The transition of the blade angles from β₁ to β₂ is sometimes referred to by those of ordinary skill in the aerodynamics arts as “blade turning.” Blade 506 a illustrates an example of blade turning. The dashed line roughly indicates the region in the blade 506 a where the “turning” occurs.

Referring to FIG. 4, the RHC-CHC boundary occurs in the middle portion 416 c of the hub 404. Generally in accordance with the present invention, blade turning occurs in the region of the middle portion 416 c of the hub 404. In an embodiment, blade turning occurs near the RHC-CHC boundary. This embodiment is illustrated in FIG. 5A where blade 506 a shows the blade turning occurring near the RHC-CHC boundary. However, blade turning can occur relatively distal the RHC-CHC boundary, but within the region of the middle portion 416 c of the hub 404.

FIG. 6 shows a portion of the schematic rendering of the impeller 502 shown in FIG. 5A. A portion of the hub 504 is shown in cross-section, as indicated by hatched lines. FIG. 6 also shows a portion of one of the fan blades 506, and another fan blade 506 a (illustrated in cross-section) oriented relative to the hub 504 in accordance with the present invention.

Utilizing a cylindrical coordinate system, let Z be the axial coordinate along the axis of rotation. Hub length L is measured as the distance downstream from a reference plane which is normal to the axis of rotation and located at the impeller hub LE (leading edge). Let R be the radial coordinate from the axis of rotation to the point of interest on the hub contour. To specify the geometric parameters or blade shape factors, shown in FIGS. 5A, 5B, and 6, the following terms are defined:

-   -   L=total axial length (i.e. length along the axis of rotation) of         impeller hub     -   L₁=axial-length of the RHC portion 504 a     -   L₂=axial-length of the CHC portion 504 b     -   R₁=hub radius at leading edge of the RHC     -   R₂=hub radius at trailing edge of the RHC     -   R₃=hub radius at trailing edge of the CHC     -   β=blade angle profile (overall turning=β₁−β₂)     -   β₁=blade angle at the leading edge     -   β₂=blade angle at the trailing edge

In an embodiment of the present invention, a hub 504 can be characterized by the following geometric relationships:

L=L ₁ +L ₂

R₂=R₃

30%<[L ₁ /L]<60%

20%<[(R ₂ −R ₁)/R ₁]<50%

10°<[β₁−β₂]<45°

Thus, R₂=R₃ expresses the idea that in an embodiment, the hub radius of the second portion 504 b of the hub 504 has a substantially constant radial measurement along its axial length. In an embodiment, the axial length L₁ of the first portion 504 a of the hub 504 can be about 30%-60% of the total length of the hub, as expressed by the relation (30%<[L₁/L]<60%). Accordingly, in an embodiment, the hub of an impeller according to the present invention has a first portion (an RHC portion) which length is at least one-quarter of the total length of the hub. In another embodiment, the hub of an impeller according to the present invention has an RHC portion which length is at least one-third of the total length of the hub. In another embodiment, the hub of an impeller according to the present invention has an RHC portion having a length that is at about one-half of the total length of the hub.

In an embodiment, the contour of the first portion 504 a of the hub 504 can have a profile that is characterized by the relation (20%<[(R₂−R₁)/R₁]<50%). In another embodiment, the contour of the first portion 504 a can have a rising arcuate or curved profile along the axial length of the first portion. FIG. 7, for example, shows in schematic fashion an embodiment in accordance with the present invention of an impeller 702 having a hub 704 and fan blades 706 (of which only one is depicted) connected to the hub. The hub 704 has an RHC portion 704 a that has an arcuate or curved profile and a CHC portion 704 b.

As discussed above, in accordance with the present invention, blade turning occurs in the proximity of the RHC-CHC boundary. In an embodiment, blade turning occurs near the RHC-CHC boundary. This embodiment is illustrated in FIGS. 5A and 5B where blade 506 a. However, in general blade turning is not restricted to the RHC-CHC boundary, but can occur within the region of the middle portion 416 c (FIG. 4) of the hub 404. In an embodiment, the leading edge portion 416 a of the hub 404 is at least 10% of the length of the hub; i.e., the length of portion 416 a is ≧10% of L. Likewise, in an embodiment, the trailing edge portion 416 b of the hub 404 is at least 10% of the length of the hub; i.e., the length of portion 416 b is ≧10% of L. Blade turning can therefore occur along about 80% of the length (L) of the hub 404 in the middle portion 416 c between the leading edge portion 416 a and the trailing edge portion 416 b.

In an embodiment, the leading edge portion 416 a of the hub 404 is about 10% to 20% of the length L of the hub. Likewise, in an embodiment, the trailing edge portion 416 b of the hub 404 is about 10% to 20% of the length L of the hub. Blade turning can therefore occur along about 60% to 80% of the length (L) of the hub 404 in the middle portion 416 c between the leading edge portion 416 a and the trailing edge portion 416 b.

In an embodiment, the blade inlet angle at the leading edge of the blade 506 a is β₁ for at least 10% of the length of the blade's camber line measured from the blade leading edge which is disposed on the leading edge portion 416 a of the hub. In an embodiment, the blade exit angle at the trailing edge of the blade 506 a is β₂ for at least 10% of the length of the blade's camber line measured from the blade trailing edge which is disposed on the trailing edge portion 416 b of the hub.

The foregoing characterizations insure in an embodiment that (a) the RHC portion 504 a is followed by the CHC portion 504 b in the downstream direction, (b) blade angle along the RHC is substantially constant, (c) blade angle along the CHC is substantially constant, and (d) most of the flow turning (or blade camber/curvature) occurs near the RHC-CHC boundary. Thus, in an embodiment, both the flow area and the blade angle (β) profile experience rapid (but continuous) changes in the vicinity of the RHC-CHC boundary. Most of the transition/reduction from β1 to β2 happens near R=R2. The cross-sectional area is shrinking along L1 and becomes substantially constant along L2.

This unique way, in which the flow is centrifuged-then-turned, renders the fan efficient while at the same time inhibits stall. Along the RHC portion of the hub the flow is centrifuged without much turning, but along the CHC portion the flow turns without being centrifuged. Due to this balance between the two mechanisms, the flow is forced to remain attached to the fan blades throughout; the would-be otherwise separated flow has nowhere to go and thus remains attached to all blade surfaces.

RHC followed by a CHC—This combination provides an effective mechanism for inhibiting stall and for increasing the aerodynamic efficiency.

Aerodynamics—This unique profile forces the flow to experience a rapid area reduction near the fan inlet followed by a constant area near the fan exit, resulting in cessation of instabilities associated with localized separated flow zones. In other words; once a weak localized stall is born it can grow and gain strength in the absence of a counter inhibiting mechanism, and when that happens the function of the fan is compromised; but our unique hub profile was able to limit the growth of flow instabilities and render the fan function normal.

Flow separation creates stall cells within the interior of the rotating impeller blades and, due to rotation, these cells also rotate in the circumferential direction but at a lower speed than the fan rotational speed, usually the cells rotate at about ½ the speed (RPM) of the fan.

Without stall 100% of the impeller flow volume performs useful work as intended; namely, capturing, pressurizing, and delivering air. But stall causes some of the flow volume to be blocked, and the blocked volume does not perform useful work.

But when stall occurs; a stall cell structure (i.e. pockets of separated low momentum flow) forms within the impeller volume and then it develops (or grows) until it occupies a portion of the total volume. Stall cell blockage is the term used to define the percentage of the volume occupied by the stall cells, weak stall may result in less than 20% blockage, and strong stall may result in 50% blockage. Blockage values range from 0% to 50% with 20% considered the threshold value (above which severe stall persists).

All fans experience flow instabilities which create some level of initial blockage. In some fan designs blockage remains low (below 20%) and the fan will function normally, in this case the LOCAL instability will not be felt and the primary performance metric, namely the P-Q (pressure-flow) curve, will not even exhibit stall. But in some designs the initial instability grows causing blockage values to be in the 20% to 50% range, in this case the P-Q curve will show stall.

The present invention provides a fan blade configuration that can (a) inhibit the growth of blockage and (b) yield high performance. In the literature blockage values are in the 0% to 50% range; but this invention prevents blockage from exceeding the threshold value, the configuration of this invention holds blockage at low levels (0% to 20%). Also, our configuration (under similar operating conditions such as speed and pressure) yields aerodynamic efficiencies that are much better than any other design (20% to 40% improvements).

In an embodiment, the blade angle β₁ of the portion of the blade attached to the RHC portion of the hub (see FIG. 5B) is substantially constant along the axial length L1 of the hub and the blade angle β₂ of the portion of the blade attached to the CHC portion of the hub is substantially constant along the axial length L2. Most of the flow turning, measured as the reduction in β, occurs near the point where L1 and L2 meet. In an embodiment the transition occurs over the middle ⅓ of the length L of the hub.

As the fan incoming flow is reduced, some of the flow may separate somewhere within the impeller blades; near the hub, tip, or in between, depending on aerodynamic design parameters such as blade solidity and camber angle profiles. In other words, the flow simply can not remain attached to the entire blade surface (i.e., all the way from the LE to the TE) all the time.

This “localized detachment process” is caused by flow separation of the suction side of the blades, producing pockets of separated (or low momentum) flow called “stall cells.” This type of instability, which occurs when some streamlines are unable to flow orderly about the blade's pressure and suction sides, may be weak or strong depending on the fan's aerodynamic design and operating conditions such as flow and speed. In general, the instability is weak at low speeds and becomes stronger and stronger with increasing speeds.

The stall cells may establish a coherent circulatory structure of their own, and when they do the stall cells rotate at about ½ the fan rotational speed and in the same direction as the fan (CW or CCW). The effects are (a) reduction in pressure because flow separation renders portions of the blades ineffective and (b) increased noise levels due to the birth of additional spinning modes (i.e., the stall cells).

In the remainder of this section a generalized principle/criteria is presented and sets forth aerodynamic conditions for preventing fan stall. By “aerodynamic conditions” is meant the selection of the correct functional relationships between the fan geometry and changes in thermodynamic properties (pressure, temperature, etc.) occurring along the flow pathway.

First, some remarks are made relating to what happens when the fan is functioning. Basically, the selected geometric parameters respond to an imposed rotation to produce changes in thermal properties such as pressure. Different geometric factors produce different thermal changes when the fan is rotating, but when three (3) relationships are satisfied the fan performance becomes excellent. First, there are two (2) mechanisms for producing pressure, the centrifugal effect created by the RHC (acting along L1 only) and flow turning due to blade camber, balancing the two is critical. Second, there are two (2) successive cross-sectional area zones, frontal zone with shrinking area and rear zone with constant area. Third, there are two (2) frequencies, an external frequency (f1, radian/sec) due to rotation and an internal frequency (f2, radian/sec) felt by the flow while it is being pressurized.

To inhibit fan stall, three aerodynamic conditions must be considered. First, 50% of the pressure should be produced via the centrifugal effect while the other 50% should be produced via flow turning. Second, the fan should have a frontal shrinking cross-sectional-area zone followed by a rear constant cross-sectional-area zone, the centrifugal effect should be accomplished within the frontal zone. Third, the external frequency due to rotation should be equal to the internal frequency associated with pressure produced within the shrinking zone.

Quantitatively, the external frequency f1=[2×Pi×RPM]/[60] and the internal frequency f2=[SQRT (Delta-P/2)]/[L1]. The total pressure-rise is Delta-P, and (per the criteria above) ½ of Delta-P must be performed over L1 (i.e. within the frontal zone).

An advantage of embodiments of the present invention is that when the frontal rapidly rising hub contour is combined with the rear constant hub contour a greater response is demonstrated. The effect of the combination, namely RHC followed by CHC, is very effective reduction of stall and may prevent stall altogether. 

1. An impeller comprising: a hub; a plurality of fan blades connected to the hub; a first part of the hub having a rising hub contour along an axial length thereof; and a second part of the hub having a substantially constant hub contour along an axial length thereof, wherein one or more of the fan blades, each, is connected to both the first part of the hub and to the second part of the hub.
 2. The impeller of claim 1 wherein a length of the first part of the hub is at least one-quarter of a total length of the hub.
 3. The impeller of claim 1 wherein a length of the first part of the hub is about 30% to 60% of a total length of the hub.
 4. The impeller of claim 1 wherein a leading edge portion of each of said one or more of the fan blades is connected to the first part of the hub and a trailing edge portion of said each of said one or more of the fan blades is connected to the second part of the hub.
 5. The impeller of claim 1 wherein for each fan blade, a first portion of said each fan blade is attached to the first part of the hub and a second portion of said each fan blade is attached to the second part of the hub, wherein about 10% of a length of said each fan blade measured from a leading edge thereof has a first blade angle, wherein about 10% of a length of said each fan blade measured from a trailing edge thereof has a second blade angle.
 6. An impeller comprising a hub and a plurality of fan blades attached to the hub, wherein a radius of the hub increases in the axial direction from a first location on the hub proximate a hub leading edge to a second location on the hub that is distal a hub trailing edge, wherein the radius does not substantially vary in the axial direction from the second location on the hub to a third location on the hub proximate the hub trailing edge.
 7. The impeller of claim 6 wherein a distance between the first location and the second location is about 30% to 60% of a distance between the first location and the third location.
 8. The impeller of claim 6 wherein each fan blade is attached to the hub between the first location and the third location.
 9. The impeller of claim 6 wherein a leading edge portion of each fan blade is attached between the first location and the second location and has a first blade angle, wherein a trailing edge portion of each fan blade is attached between the second location and the third location and has a second blade angle different from the first blade angle.
 10. The impeller of claim 9 wherein blade turning in said each fan blade occurs proximate the second location on the hub.
 11. The impeller of claim 6 wherein the radius of the hub increases between the first location and the second location in linear fashion.
 12. An impeller comprising a hub having a first segment and a second segment, the impeller further comprising a plurality of fan blades attached symmetrically about the hub, each fan blade being attached to a part of the first segment of the hub and a part of the second segment of the hub, the first segment of the hub having a rising hub profile, the second segment of the hub having a substantially constant hub profile.
 13. The impeller of claim 12 wherein a radius of the first segment of the hub varies along the axial direction thereof, wherein a radius of the second segment of the hub remains substantially the same along the axial direction thereof.
 14. The impeller of claim 13 wherein the radius of the first segment of the hub varies in linear fashion.
 15. The impeller of claim 12 wherein a length of the first segment is about 30% to 60% of the sum of the length of the first segment and a length of the second segment.
 16. The impeller of claim 12 wherein for each fan blade, a blade inlet angle is β₁ for a portion of said each fan blade equal in length to about 10% of a length of said each blade measured from its leading edge, and a blade exit angle is β₂ for a portion of said each fan blade equal in length to about 10% of the length of said each blade measured from its trailing edge.
 17. The impeller of claim 16 wherein a length of a camber line of said each blade constitutes the length of said each blade.
 18. The impeller of claim 16 wherein for said each fan blade, blade turning occurs in a middle portion of said hub between a first portion of said hub and a second portion of said hub.
 19. A fan comprising an impeller of claim 12 disposed in a fan housing and connected to a fan motor. 